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image of Journal of Crustacean Biology

ABSTRACT Thoracopod development was analyzed for 25 species from 18 genera among the Cyclopidae. One species from each of 7 genera of presumed older cyclopoids, 5 poecilostomatoid genera, 3 harpacticoid genera, 5 calanoid genera, and a siphonostomatoid were studied to help establish ancestral patterns of thoracopod development. Ancestral character states were inferred from the presence of identical states in species from presumed older families, from the presence of states which show diverse and frequent occurrence among the Copepoda, or from the presence of states shared by serially homologous structures presumed to be determined by the same pleiotropic regulatory process. Developmental patterns of the 4 swimming legs were assumed to result from the action of 2 different regulatory processes. A pleiotropic process acting early in development determines the morphology of all 8 rami together and results in 3 states among cyclopids, ancestral, and the independently derived delayed and truncated patterns. A second set of up to 8 regulatory processes acts later and one of the 8 determines the morphology of each individual ramus. The ancestral states of the resulting individual rami are, by default, the morphology that results from the pleiotropic regulatory process. Variations in developmental patterns were used to generate a phylogenetic hypothesis. Cyclopids have separated into a lineage of 10 species which has delayed the development of the swimming legs or has modified some individual rami from the delayed condition, and a lineage of 8 species which has truncated the development of the swimming legs or has modified some individual rami from that truncated condition. Four species have retained the ancestral process regulating swimming-leg development and 3 species have modified development of some individual rami from the ancestral condition.


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